Synchronous drive apparatus and methods

ABSTRACT

A synchronous drive apparatus and method, wherein the apparatus comprises a plurality of rotors comprising at least a first and a second rotor. The first rotor has a plurality of teeth for engaging the engaging sections of an elongate drive structure, and the second rotor has a plurality of teeth for engaging the engaging section of the elongate drive structure. A rotary load assembly is coupled to the second rotor. The elongate drive structure engages about the first and second rotors. The first rotor is arranged to drive the elongate drive structure and the second rotor is arranged to be driven by the elongate drive structure. One of the rotors has a non-circular profile having at least two protruding portions alternating with receding portions. The rotary load assembly is such as to present a periodic fluctuating load torque when driven in rotation, in which the angular positions of the protruding and receding portions of the non-circular profile relative to the angular position of the second rotor, and the magnitude of the eccentricity of the non-circular profile, are such that the non-circular profile applies to the second rotor an opposing fluctuating corrective torque which reduces or substantially cancels the fluctuating load torque of the rotary load assembly.

[0001] The present application claims priority to U.S. ProvisionalApplication of Gajewski, Application Nos. 60/333,118, filed Nov. 27,2001 and 60/369,558, filed Apr. 24, 2002, the entirety of which arehereby incorporated into the present application by reference.

FIELD OF INVENTION

[0002] The present invention relates to a synchronous drive apparatus, amethod of operating a synchronous drive apparatus and a method ofconstructing a synchronous drive apparatus. The invention relates to theelimination or reduction of mechanical vibrations, in particular but notexclusively in internal combustion engines.

BACKGROUND OF INVENTION

[0003] Synchronous drive systems, such as timing belt-based systems, arewidely used in motor vehicles, as well as in industrial applications. Inmotor vehicles, for example, timing belts or chains are used to drivethe camshafts that open and close the engine intake and exhaust valves.Also other devices such as water pumps, fuel pumps etc. can be driven bythe same belt or chain.

[0004] Internal combustion engines produce many types of mechanicalvibrations during their operation, and these vibrations are usuallytransmitted through the timing belt or chain in the synchronous drivesystem. A particularly intense source of mechanical vibrations is givenby the intake and exhaust valves and the camshafts that open and closethose intake and exhaust valves. Opening and closing the intake andexhaust valves leads to a type of vibration known as torsionalvibration. When the frequency of these vibrations is close to naturalfrequency of the drive, system resonance occurs. In resonance thetorsional vibrations and the span tension fluctuations are at theirmaximum.

[0005] As flexible mechanical structures, timing belts and chains areparticularly susceptible to the deleterious effects of mechanicalvibrations. Mechanical vibrations transmitted through the timing belt orchain cause fluctuations in belt or chain tension, which can lead toincreased wear and reduced belt or chain life. Vibrations may also causetiming errors, and result in undesirable amounts of noise.

[0006] Conventional techniques to attenuate the vibrations includeincreasing the tension on the belt or chain and installing camshaftdampers. Camshaft dampers connect a source of inertia to a camshaftsprocket by a vibration-absorbing rubber or silicone. However,increasing the belt or chain tension increases the noise level andreduces the useful life of the belt or chain. Installing camshaftdampers is also an undesirable solution, because of their cost and/orbecause of lack of space.

[0007] In DE-A-195 20 508 (Audi AG), there is disclosed a wrapped beltdrive for an internal combustion engine, the timing belt being wrappedaround two driven pulleys coupled to the camshaft of the engine, and onedrive pulley coupled to the crankshaft of the engine. The objective ofthe invention is to counter the torsional vibrations which are found insuch belt drives. It is proposed to provide an additional torsionalvibration through which the critical resonance can be moved to a rangewhere it can either be tolerated, or does not arise. It is proposed inthe citation to produce torsional vibrations by an “out of round”pulley, which is shown as consisting of one of the camshaft pulleys. Theout of round pulley which is shown has four protruding portions and fourreceding portions arranged regularly around the pulley. It is said thatthe variations in the pulley profile introduce torsionals to the timingbelt at the incoming or outgoing spans of the driven pulleys, which aresuperimposed on the dynamics of the combustion engine, and thus shift oreliminate the critical resonance range. A figure is shown which is saidto show a graph of torsional vibrations of the timing drive in degreescamshaft over the RPM of the crankshaft. The total amplitude is shown,and also the dominant vibration of the second order and the lessrelevant vibrations of the fourth order are shown. A single example of amagnitude of eccentricity of an out of round pulley is given, but noteaching is given as to how to select the magnitude of the eccentricity,and the angular alignment of the out of round rotor relative to theother rotors, for any given conditions of type of engine, type of drivebelt, and type of load. As has been mentioned, the objective of theinvention in the citation is to counter the torsional vibrations in thebelt drive, and not to deal with the source of the vibrations.

[0008] In Japanese Utility Model JP 62-192077 (Patent Bulletin No. HEI1-95538) of 1987 (Hatano et al/Mitsubishi), there is disclosed a tensionequalising driving device which transmits the rotation of a drive pulleyto a driven pulley by a belt drive such as a timing belt in an internalcombustion engine. There is shown a timing belt arrangement in which atoothed pulley of the drive shaft of a camshaft is driven by an ovaltiming belt driving sprocket connected to the drive shaft of an internalcombustion engine. The teaching of the citation is that the drive pulleyis made oval in shape so as to give the drive belt a tension fluctuationwith a phase opposite to that of the tension fluctuation in the beltproduced by the rotation of the internal combustion engine. It is saidthat the drive pulley is installed in such a way that it gives the drivebelt a tension fluctuation with a phase opposite to that of the tensionfluctuation of the belt already present. The oval drive sprocket is saidto be a tension equalising device, and is provided to equalise thetension in the drive belt. A figure is shown of a graph illustrating thetension caused by the valve train torque and the tension caused by thetension equalising device (the oval drive sprocket), the two tensionsbeing shown of the same magnitude and opposite phase. There is nospecific teaching given as to how to determine the magnitude of theeccentricity of the oval drive pulley, nor how to relate the angularposition of the drive pulley to the camshaft pulley which is driven bythe belt. In addition, as discussed in Japanese Application No. HEI9-73581 (Patent Bulletin No. HEI 10-266868) of 1997 (Kubo/Mitsubishi),it was subsequently determined by the Applicant in JP 62-192077 (HEI1-95538) that the use of an oval sprocket as a crank sprocket has anumber of difficulties and problems and is thus not desirable.

SUMMARY OF INVENTION

[0009] In accordance with the present invention in a first aspect, thereis provided a synchronous drive apparatus, comprising a continuous-loopelongate drive structure having a plurality of engaging sections. Aplurality of rotors comprising at least a first and a second rotor,wherein the first rotor has a plurality of teeth for engaging theengaging sections of the elongate drive structure, and the second rotorhas a plurality of teeth for engaging the engaging section of theelongate drive structure. A rotary load assembly is coupled to thesecond rotor. The elongate drive structure engages about the first andsecond rotors. The first rotor is arranged to drive the elongate drivestructure and the second rotor is arranged to be driven by the elongatedrive structure. One of the rotors has a non-circular profile having atleast two protruding portions alternating with receding portions., Therotary load assembly is such as to present a periodic fluctuating loadtorque when driven in rotation, in which the angular positions of theprotruding and receding portions of the non-circular profile relative tothe angular position of the second rotor, and the magnitude of theeccentricity of the non-circular profile, are such that the non-circularprofile applies to the second rotor an opposing fluctuating correctivetorque which reduces or substantially cancels the fluctuating loadtorque of the rotary load assembly.

[0010] In preferred forms of the apparatus, the non-circular profile issuch as to produce the opposing fluctuating corrective torque byperiodic elongation and contraction of the spans of the elongate drivestructure adjoining the rotor on which the non-circular profile isformed. The elongate drive structure has a drive span on the tight sideof the rotor on which the non-circular profile is formed, the angularposition of the non-circular profile being within +/−15 degrees(preferably within +/−5 degrees) of an angular position for which amaximum elongation of the drive span coincides with a peak value of thefluctuating load torque of the rotary load assembly. Most preferably theangular position of the non-circular profile is that for which a maximumelongation of the drive span substantially coincides with a peak valueof the fluctuating load torque of the rotary load assembly.

[0011] Also in preferred forms of the apparatus, the magnitude of theeccentricity of the non-circular profile is such that the fluctuatingcorrective torque has an amplitude in the range of 70% to 110%(preferably in the range 90% to 100%) of the amplitude of thefluctuating load torque at a predetermined selected set of operatingconditions of the synchronous drive apparatus. Most preferably, theamplitude of the fluctuating corrective torque is substantially equal tothe amplitude of the fluctuating load torque.

[0012] In this specification, unless otherwise indicated, the termamplitude of a periodically varying item means peak-to-peak amplitude.

[0013] Thus, the magnitude of the eccentricity of the non-circularprofile is determined with reference to the amplitude of the fluctuatingload torque of the rotary load assembly. In some arrangements theamplitude of the fluctuating load torque may be substantially constant,and in other arrangements the amplitude of the fluctuating load torquemay vary. Where the amplitude of the fluctuating load torque isconstant, the magnitude of the eccentricity is determined with referenceto that substantially constant amplitude of fluctuating load torque.Where the amplitude of the fluctuating load torque varies, the valuethereof which is used to determine the magnitude of the eccentricitywill be selected according to the operating conditions in which it isdesired to eliminate or reduce the unwanted vibrations. For examplewhere the fluctuating load torque of the rotary load assembly varies,the eccentricity may be determined with reference to the amplitude ofthe fluctuating load torque when determined at conditions such that itis a maximum, or for example when determined at the natural resonancefrequency of the apparatus. For example in a diesel internal combustionengine, the most troublesome region for vibration may be at the maximumfuel delivery by the fuel pump. In these conditions, the eccentricity isdetermined with reference to the amplitude of the fluctuating loadtorque when determined at these conditions. Similarly in a petrol orgasoline internal combustion engine, the most troublesome region may beat the region of natural resonance of the timing drive, and in such,acase the eccentricity is determined with reference to such conditions.

[0014] It is to be appreciated that the invention finds application inmany forms of synchronous drive apparatus other than in internalcombustion engines. Also, the non-circular profile may be provided inmany different locations within the drive apparatus. For example anon-circular profile may be provided on the first rotor (which drivesthe elongate drive structure), and/or on the second rotor (which isdriven by the elongate drive structure), and/or may be provided on athird rotor, for example an idler rotor urged into contact with thecontinuous loop elongate drive structure.

[0015] However, the invention finds particular use when installed in aninternal combustion engine and the first rotor comprises a crankshaftsprocket. In some arrangements the internal combustion engine is adiesel engine, and the rotary load assembly comprises a rotary fuelpump. As has been mentioned in such arrangements, it may be arrangedthat the magnitude of the eccentricity of the non-circular profile issuch that the fluctuating corrective torque has an amplitudesubstantially equal to the amplitude of the fluctuating load torque whendetermined at conditions of maximum delivery of the fuel pump. In otherarrangements, the internal combustion engine may be a petrol or gasolineengine and the rotary load assembly may be a camshaft assembly.

[0016] In determining the angular position of the non-circular profile,consideration may be given to various reference parameters of theprofile and the rotor on which it is formed. In some arrangements thenon-circular profile has at least two reference radii, each referenceradius passing from the centre of the rotor on which the non-circularprofile is formed and through the centre of a protruding portion of thenon-circular profile, and the angular position of the non-circularprofile is related to a reference direction of the rotor on which thenon-circular profile is formed, the reference direction being thedirection of the hub load force produced by engagement of the elongatedrive structure with that rotor. The angular position of thenon-circular profile is such that, when the fluctuating load torque ofthe rotary load assembly is at a maximum, the annular position of areference radius is preferably within a range of 90° to 180° from thereference direction taken in the direction of rotation of the rotor onwhich the non-circular profile is formed. Preferably, the rangecomprises a range of 130° to 140°. Most preferably, the angular positionof the reference radius is substantially at 135° from the referencedirection taken in the direction of rotation of the rotor on which thenon-circular profile is formed.

[0017] It will be appreciated that many different forms of non-circularprofile may be provided, for example a generally oval profile, or aprofile having three or four protruding portions arranged regularlyaround the rotor. The choice of profile will depend upon othercomponents of the synchronous drive apparatus. Examples which may beprovided include the following, namely: the internal combustion engineis a 4-cylinder inline combustion engine and the crankshaft sprocket hasan oval contoured profile; the internal combustion engine is a4-cylinder inline combustion engine and the camshaft sprocket has agenerally rectangular contoured profile; the internal combustion engineis a 4-cylinder inline combustion engine, and the camshaft sprocket hasa generally rectangular contoured profile and the crankshaft sprockethas an oval contoured profile; the internal combustion engine is a3-cylinder inline combustion engine and the camshaft sprocket has agenerally triangular contoured profile; the internal combustion engineis a 6-cylinder inline combustion engine and the crankshaft sprocket hasa generally triangular contoured profile; the internal combustion engineis a 6-cylinder V6 combustion engine and the camshaft sprocket has agenerally triangular contoured profile; the internal combustion engineis an 8-cylinder V8 combustion engine and the camshaft sprocket has agenerally rectangular contoured profile; or the internal combustionengine is a 2-cylinder combustion engine and the camshaft sprocket hasan oval contoured profile.

[0018] In most embodiments of the invention as set out above, theprotruding portions and receding portions will be generally of the samemagnitude, giving a regular non-circular profile. However depending uponthe circumstances of the torsional vibrations to be removed, anon-regular profile may be provided. Furthermore, the protrudingportions referred to above may constitute major protruding portions andthe receding portions constitute major receding portions, and thenon-circular profile may include additional minor protruding portions oflesser extent than the major protruding portions. These minor protrudingportions may be adapted to produce additional, minor, fluctuatingcorrective torque patterns in the torque applied to the second rotor,for the purpose of reducing or substantially cancelling subsidiary orderfluctuating load torque presented by the rotary load assembly, inparticular for example in order to reduce or substantially cancel fourthorder fluctuating load torques presented by the rotary load assembly.

[0019] It is to be appreciated that where features of the invention areset out herein with regard to apparatus according to the invention, suchfeatures may also be provided with regard to a method according to theinvention (namely a method of operating a synchronous drive apparatus,or a method of constructing a synchronous drive apparatus), and viceversa.

[0020] In particular, there is provided in accordance with anotheraspect of the invention a method of operating a synchronous driveapparatus which comprises a continuous-loop elongate drive structurehaving a plurality of engaging sections. A plurality of rotors comprisesat least a first and a second rotor. The first rotor has a plurality ofteeth engaging the engaging sections of the elongate drive structure,and the second rotor has a plurality of teeth engaging the engagingsection of the elongate drive structure. A rotary load assembly iscoupled to the second rotor. One of the rotors has a non-circularprofile having at least two protruding portions alternating withreceding portions. The rotary load assembly presents a periodicfluctuating load torque when driven in rotation.

[0021] The method comprises the steps of engaging the elongate drivestructure about the first and second rotors, driving the elongate drivestructure by the first rotor, and driving the second rotor by theelongate drive structure, and applying to the second rotor by means ofthe non-circular profile an opposing fluctuating corrective torque whichreduces or substantially cancels the fluctuating load torque of therotary load assembly.

[0022] In accordance with yet another aspect of the invention, there maybe provided a method of constructing a synchronous drive apparatus,comprising:

[0023] (i) assembling components comprising a continuous-loop elongatedrive structure having a plurality of engaging sections, a plurality ofrotors comprising at least a first and a second rotor, the first rotorhaving a plurality of teeth for engaging the engaging sections of theelongate drive structure, and the second rotor having a plurality ofteeth for engaging the engaging section of the elongate drive structure,and a rotary load assembly coupled to the second rotor; and

[0024] (ii) engaging the elongate drive structure about the first andsecond rotors, the first rotor being arranged to drive the elongatedrive structure and the second rotor being arranged to be driven by theelongate drive structure, and one of the rotors having a non-circularprofile having at least two protruding portions alternating withreceding portions, the rotary load assembly being such as to present aperiodic fluctuating load torque when driven in rotation; and

[0025] (iii) determining the angular positions of the protruding andreceding portions of the non-circular profile relative to the angularposition of the second rotor, and the magnitude of the eccentricity ofthe non-circular profile, to be such that the non-circular profileapplies to the second rotor an opposing fluctuating corrective torquewhich reduces or substantially cancels the fluctuating load torque ofthe rotary load assembly.

[0026] In a preferred form of the method of constructing the synchronousdrive apparatus, the method includes:

[0027] (i) arranging the non-circular profile to produce the opposingfluctuating corrective torque by periodic elongation and contraction ofthe spans of the elongate drive structure adjoining the rotor on whichthe non-circular profile is formed, the elongate drive structure havinga drive span between the rotor on which the non-circular profile isformed and the second rotor, the drive span being positioned on thetight side of the rotor on which the non-circular profile is formed; and

[0028] (ii) determining the angular positions of the protruding andreceding portions of the non-circular profile by arranging the angularposition of the non-circular profile to be within +/−15 degrees of anangular position for which a maximum elongation of the drive spancoincides with a peak value of the fluctuating load torque of the rotaryload assembly.

[0029] Also in a preferred form of the invention the method ofconstructing a synchronous drive apparatus includes determining themagnitude of the eccentricity of the non-circular profile is determinedby the following steps:

[0030] (i) measuring the amplitude of the fluctuating load torque of therotary load assembly at a predetermined selected set of operatingconditions of the synchronous drive apparatus;

[0031] (ii) calculating the required amplitude of periodic elongationand contraction of the drive span by the following formula:$L = \frac{T}{rk}$

[0032] L=the amplitude of the periodic elongation and contraction of thesaid drive span;

[0033] T=the amplitude of the fluctuating load torque of the rotary loadassembly at a predetermined selected set of operating conditions of thesynchronous drive apparatus;

[0034] r=the radius of the second rotor:

[0035] k=the stiffness coefficient of the elongate drive structuredefined as $\begin{matrix}{k = \quad {dF}} \\{\quad {d\quad L}} \\\quad\end{matrix}$

[0036]  where dF is the force required to produce an increase of lengthdL in the length of the structure.

[0037] (iii) producing and recording data to relate empirically a seriesof values of (a) the divergence from circular of the protruding andreceding portions of the non-circular profile and (b) the resultingamplitude of the periodic elongation and contraction of the drive span;and

[0038] (iv) selecting from the data the corresponding eccentricity togive the required amplitude of the periodic elongation and contractionof the drive span.

[0039] The present invention arises from an understanding that the bestway to eliminate or reduce torsional vibrations in a synchronous drivesystem is to arrange a non-circular profile on one of the rotors whichis such as to cancel or reduce the fluctuating load torque in the loadassembly, rather than trying to cancel or reduce the varying tension inthe continuous loop drive structure, as was attempted in the prior art.Indeed it is found essential to provide a varying tension in theelongate drive structure, in order to cancel or reduce the fluctuatingload torque in the load assembly. The present invention allows thecancellation, or reduction, of the source of the torsional excitation,rather than endeavouring to deal with the effects of torsionals bycancelling variations in tension in the elongate drive structure.

[0040] Thus although it has been known to provide a non-circular profileon one of the rotors in a synchronous drive assembly, the methods chosento determine the magnitude of the eccentricity, and the timing of theprotruding and receding portions of the non-circular profile, have notbeen such as to produce the required result. By way of example, in atypical internal combustion engine, if the eccentricity is chosen suchas to try to equalise the tension in a drive belt, the eccentricity willtypically be considerably too great to cancel the torsional vibrationsin the load assembly. In a typical international combustion engine,there will be a resonant frequency at, say, 2000 to 2500 rpm. If theeccentricity of the non-circular profile is chosen to attempt to cancelany tension variation in the drive belt in the region of resonance, thentypically the eccentricity will be set at much more tension than isrequired to cancel the vibrations. The result will be excessive wear inthe drive belt and the various sprockets, and also the system will notbe successful in reducing vibration.

[0041] Considering another manner in which the prior art arrangementswere deficient, it is important to arrange the timing (translated intoangular position) of the non-circular profile, to be correctly relatedto the timing (translated into angular positioning) of the fluctuationsin load torque in the load assembly. Conveniently the relative timing ofthe non-circular profile and the fluctuating load torque of the rotaryload assembly is determined in relation to a periodic elongation andcontraction of a drive span of the elongate drive structure between thefirst and second rotors on the tight side of the first rotor. The mostpreferable arrangement in accordance with the invention is that theangular position of the non-circular profile is that for which a maximumelongation of the drive span of the elongate drive structuresubstantially coincides with a peak value of the fluctuating load torqueof the rotary load assembly. However, the invention can providesubstantial reduction in vibration if the timing is set within a rangeof plus/minus 15° of the preferred angular position. A particularlypreferred range is plus/minus 5° of the preferred angular position.

[0042] In contrast, in the prior art it has been attempted to set theeccentricity of the non-circular profile with reference to the tensionin the elongate drive structure. However in a typical internalcombustion engine the peak tension in the drive belt varies in itstiming according to the region of the rpm range which is examined.Typically the peak tension in the drive belt occurs at one timing stagefor the resonant frequency of the engine, and occurs at an earliertiming in the cycle for the rev range below resonance, and occurs at alater part of the timing cycle for the region of the rev range above theresonant condition. Thus, depending upon which conditions are selectedin the prior art in order to attempt to equalise the tension in thedrive belt, the timing of the eccentricity of the non-circular profilemay be ahead of, or may lag behind, the preferred position forcancelling the fluctuating load torque in the load assembly.

[0043] Thus to summarise, the present invention provides for the correctselection of the eccentricity and the timing of the non-circularprofile, to be that which most advantageously cancels or reduces thefluctuating load torque in the load assembly.

DESCRIPTION OF THE DRAWINGS

[0044] Embodiments of the invention will now be described by way ofexample with reference to the accompanying drawings in which:

[0045]FIG. 1 is a schematic illustration of a synchronous driveapparatus for a motor vehicle internal combustion engine, embodying theinvention;

[0046]FIG. 2 is an enlarged view of the crankshaft sprocket shown inFIG. 1;

[0047]FIG. 3 is a schematic illustration of the synchronous driveapparatus of an internal combustion engine in DOHC engine configuration;

[0048]FIG. 4a shows a graph of a fluctuating load torque at the camshaftof an SOHC internal combustion engine and a fluctuating correctivetorque generated by an oval crankshaft sprocket illustrated in FIGS. 1and 2, all graphs being taken over one crankshaft revolution;

[0049]FIG. 4b shows a graph of a fluctuating load torque which arisesfrom the intake cam of an DOHC internal combustion engine, a fluctuatingload torque which arises from the exhaust cam, and a fluctuatingcorrective torque generated by an oval crankshaft sprocket in the engineillustrated in FIG. 3, all graphs being taken over one crankshaftrevolution;

[0050]FIGS. 5a to 5 d show different combinations of crankshaft andcamshaft sprockets embodying the invention in 4-cylinder and 3-cylinderengines;

[0051]FIGS. 6a to 6 d show different combinations of crankshaft andcamshaft sprockets embodying the invention in 6-cylinder, 8-cylinder and2-cylinder engines;

[0052]FIG. 7a is a graph illustrating the magnitude of torsionalvibrations in an internal combustion engine at different engine speeds,the vertical axis indicating the amplitude of torsional vibrations indegrees of movement of the camshaft, and the horizontal axis indicatingengine speed in rpm, the graph indicating the situation in a knownengine, having a round crankshaft sprocket;

[0053]FIG. 7b is a graph illustrating the magnitude of torsionalvibrations in an internal combustion engine at different engine speeds,the vertical axis indicating the amplitude of torsional vibrations indegrees of movement of the camshaft, and the horizontal axis indicatingengine speed in rpm, the graph indicating the situation for asynchronous drive apparatus embodying the invention, utilising an ovalcrankshaft sprocket;

[0054]FIG. 8a is a graph illustrating the magnitude of tensions in aninternal combustion engine at different engine speeds, the vertical axisindicating the amplitude of the belt tension, and the horizontal axisindicating engine speed in rpm, the graph indicating the situation in aknown engine, having a round crankshaft sprocket;

[0055]FIG. 8b is a graph illustrating the magnitude of tensions in aninternal combustion engine at different engine speeds, the vertical axisindicating the amplitude of the belt tension, and the horizontal axisindicating engine speed in rpm, the graph indicating the situation for asynchronous drive apparatus embodying the invention, utilising an ovalcrankshaft sprocket;

[0056]FIGS. 9a and 9 b show respectively the fluctuations in tension inthe drive belt over one revolution of the crankshaft at 1500 RPM, for anengine according to the prior art, having a round crankshaft sprocket,FIGS. 9a and 9 b showing respectively the belt tension variations on thetight side and the slack slide of the crankshaft sprocket respectively;

[0057]FIGS. 10a and 10 b show respectively the fluctuations in tensionin the drive belt over one revolution of the crankshaft at 2500 RPM, foran engine according to the prior art, having a round crankshaftsprocket, FIGS. 10a and 10 b showing respectively the belt tensionvariations on the tight side and the slack slide of the crankshaftsprocket respectively;

[0058]FIG. 11 show respectively the fluctuations in tension in the drivebelt over one revolution of the crankshaft at 3500 RPM, for an engineaccording to the prior art, having a round crankshaft sprocket, FIGS.11a and 11 b showing respectively the belt tension variations on thetight side and the slack slide of the crankshaft sprocket respectively;

[0059]FIG. 12 is a three-dimensional graph showing the distribution ofcamshaft torsional vibrations in a known internal combustion enginehaving a round crankshaft sprocket, in which the X-axis indicatesvarious harmonic orders of vibration, the Y-axis indicates engine speedin RPM, and the Z-axis indicates the amplitude of the camshaft torsionalvibrations;

[0060]FIG. 13 is a three-dimensional graph showing the distribution ofcamshaft torsional vibrations in an engine embodying the invention andhaving an oval crankshaft sprocket, in which the X-axis indicatesvarious harmonic orders of vibration, the Y-axis indicates engine speedin RPM, and the Z-axis indicates the amplitude of the camshaft torsionalvibrations;

[0061]FIG. 14a shows a graph of fluctuating load torque on a rotary loadassembly such as a camshaft;

[0062]FIG. 14b shows how a non-circular profile 19 may be derived tocancel the torque fluctuations of FIG. 14a, in an embodiment of theinvention; and

[0063]FIGS. 15, 16 and 17 show a computer generated virtualrepresentation of an oval crankshaft profile embodying the invention,the profile being stepped on by an angular advance of one tooth in FIG.16 relative to FIG. 15, and in FIG. 17 relative to FIG. 16.

DESCRIPTION OF THE INVENTION

[0064]FIG. 1 is a diagrammatic representation of a synchronous driveapparatus for a motor vehicle internal combustion engine, embodying theinvention. The apparatus comprises a continuous loop elongate drivestructure 10, first and second rotors 11 and 12, and further rotors 13,14 and 17. The continuous loop elongate drive structure 10 is providedby a conventional timing belt having teeth 15 together with interveningvalleys which constitute a plurality of engaging sections of thecontinuous loop elongate drive structure. Each rotor 11 and 12 isprovided by a sprocket having a plurality of teeth 16 for engaging thevalleys between the teeth 15 of the timing belt 10. The sprocket 11 iscoupled to the crankshaft (not shown) of an internal combustion engine,and the sprocket 12 is coupled to a rotary load assembly (not shown)which is constituted by a camshaft 26 of the internal combustion engine.The timing belt 10 is engaged about the first and second rotors 11 and12, the first rotor 11 being arranged to drive the belt 10 and thesecond rotor 12 being arranged to be driven by the belt 10. The rotor 14also has teeth 16 and consists of a sprocket for driving other elementsof the internal combustion engine, such as a water pump, and the rotor13 is preferably for a belt tensioner bearing on a non-toothed side ofthe timing belt 10, to tension the belt in known manner. Rotor 17 ispreferably for a fixed idler pulley bearing on the non-toothed side oftiming belt 10.

[0065] In a known form of a synchronous drive apparatus, the crankshaftsprocket would have a circular profile. In such a case, the synchronousdrive apparatus is prone to vibrations, known as torsional vibrations,which arise from the opening and closing of the intake and exhaustvalves of the internal combustion engine by the overhead camshaft. Thesource of the excitations is illustrated in FIGS. 4a and b. FIG. 4aillustrates the fluctuating load torque 103 applied to the camshaft in aSOHC engine and FIG. 4b illustrates the same for a DOHC engine. FIG. 4bshows the variation of camshaft torque over a single cycle of theengine, indicating how the intake torque shown by the curve 101 varieswith degrees of rotation of the engine, and how the exhaust torqueprofile 102 varies in the same way.

[0066] In accordance with the embodiment of the present invention shownin FIG. 1 for a SOHC engine, the crankshaft sprocket 11 has anon-circular profile (as shown in exaggerated form in FIG. 2) indicatedgenerally by reference numeral 19. The non-circular profile 19 is, inthe particular embodiment described, an oval having a major axis 20 anda minor axis 21. The profile 19 has two protruding portions 22 and 23and has two receding portions 24 and 25.

[0067] The provision of the oval profile 19 on the sprocket 11 as shownin FIG. 2, generates a fluctuating corrective torque, which is appliedby the belt 10 to the second rotor 12. This fluctuating correctivetorque is shown at 104 in FIG. 4a. In the preferred situation, the totalfluctuating load torque 103 is opposed by the overall corrective torque104. Preferably the corrective torque 104 is 180° out of phase with theoverall load torque 103, and the peak to peak amplitude of thefluctuating corrective torque 104 is made equal to the peak to peakamplitude of the overall fluctuating load torque 103.

[0068] In accordance with the embodiment of the invention using the ovalprofile 19 shown in FIG. 2, the angular positions of the protruding andreceding portions 22 to 24 of the non-circular profile 19 relative tothe angular position of the second rotor 12, and the magnitude of theeccentricity of the non-circular profile 19, are such that thenon-circular profile 19 applies to the second rotor 12 an opposingfluctuating corrective torque 104 which substantially cancels thefluctuating load torque 103 of the rotary load assembly 26.

[0069] The determination of the timing and magnitude of the eccentricityof the non-circular profile 19 will now be described in more detail. InFIG. 1 the spans between the various rotors are indicated as 10A betweenrotor 12 and rotor 14, 10B between rotor 14 and rotor 11, 10C betweenrotor 12 and rotor 13, and 10D between rotor 13 and rotor 17 and 10Ebetween rotor 17 and rotor 11. The span between the first rotor 11 andthe second rotor 12, indicated as 10A, 10B, is referred to as the drivespan between the two rotors, it being positioned on the tight side ofthe first rotor 11 on which the non-circular profile 19 is formed. Thespan between the first rotor 11 and second rotor 12 which is indicatedas 10C, 10D, 10E is referred as the slack side, although of course thebelt is under tension on both sides. The torsional vibrations to beeliminated are formed by the fluctuating load torque on the rotary loadassembly (the camshaft 26) and in accordance with the present inventionthis is reduced or substantially cancelled by the application of anopposing fluctuating corrective torque to the camshaft 26 by means ofthe timing belt 10. The opposing fluctuating corrective torque isproduced by the non-circular profile 19 by periodic elongation andcontraction of the spans 10A 10B and 10C 10D 10E, adjoining the rotor 11on which the non-circular profile is formed. In preferred forms of theinvention, the angular position of the non-circular profile 19 is set asclosely as possible to be that for which a maximum elongation of thedrive span 10A 10B substantially coincides with a peak value of thefluctuating load torque of the camshaft 26. It may not always bepossible to arrange this exactly, and advantage is obtained inaccordance with the invention if the angular position of thenon-circular profile is within +/−15 degrees of the preferred angularposition, more preferably within +/−5 degrees.

[0070] With regard to the particular case illustrated, and referring toFIGS. 1 and 2, the oval profile 19 has two reference radii 20 a and 20b, which together form the major axis 20 of the oval. Each referenceradius 20 a, 20 b passes from the centre of the rotor 11 and through thecentre of the respective protruding portion 22, 23. The angular positionof the non-circular profile 19 is related to a reference direction ofthe rotor 11, the reference direction being the direction of a vector orimaginary line 27 that bisects the angle or sector of wrap of thecontinuous loop drive structure 10 around the rotor 11. This vector thatbisects the angle of wrap is in the same direction as the hub load forceproduced by engagement of the belt 10 with the rotor 11 when the beltdrive system is static. It should be appreciated, however, that the hubload force direction changes dynamically during operation of the beltdrive system. The timing of the non-circular profile 19 is set to besuch that, at the time when the fluctuating load torque on the secondrotor 12 is at a maximum, the angular position of the reference radius20 a is within a range of 90° to 180° from the reference direction ofthe angle of wrap bisection 27, taken in the direction of rotation ofthe rotor 11, preferably within a range of 130° to 140°. Assuming thatthe assembly of FIG. 1 is shown at the instant when the fluctuating loadtorque on the second rotor 12 is at a maximum, then the preferred timingof the non-circular profile 19 is as shown in FIG. 1, namely that theangle between the reference radius 20 a and the bisection direction 27is 135°, as indicated by the angle θ.

[0071] It is to be appreciated that in this specification, where theterm “reference radius” is used for a non-circular profile 19, thereference parameter measured is the radius of a notional circle passingthrough the associated protruding portion, and is not a radius of theentire profile, since this entire profile is essentially non-circular.The term reference radius is used merely to indicate the distancebetween the centre of the axis of the rotor on which the profile isformed, to the maximum extent of the profile at the relevant protrudingportion.

[0072] Consideration will now be given to the determination of themagnitude of the eccentricity of the non-circular profile 19 in thespecific embodiment shown. In summary, the magnitude of the eccentricityof the profile 19 is preferably set to be such that the fluctuatingcorrective torque 104 shown in 4 a has an amplitude substantially equalto, and phase substantially opposite to, the amplitude of thefluctuating load torque 103 shown in FIG. 4a. However advantage is stillfound in embodiments where the amplitude of the fluctuating correctivetorque 104 is in the range of 75% to 110% of the amplitude of thefluctuating load torque 103, more preferably in the range 90% to 100%.Where the fluctuating load torque 103 has a substantially constantamplitude over the rev range of the engine, the amplitude of thecorrective torque 104 is merely made substantially equal to the constantamplitude of the fluctuating load torque.

[0073] The practical steps of determining the magnitude of theeccentricity may be as follows. First the amplitude of the fluctuatingload torque 103 of the camshaft 26 is measured at the selected set ofoperating conditions, in this case at the maximum amplitude of thefluctuating load torque. Next there is calculated the required amplitudeof period elongation and contraction of the drive span 10 a, 10 b by thefollowing formula: $L = \frac{T}{rk}$

[0074] where:

[0075] L=the amplitude of the periodic elongation and contraction of thedrive span which is required;

[0076] T=the amplitude of the fluctuating load torque of the camshaft26, which has been measured at maximum amplitude;

[0077] r=the radius of the second rotor 12: and

[0078] k=the stiffness coefficient of the belt 10.

[0079] The stiffness coefficient k is obtained from the formula$\begin{matrix}{k = \quad {dF}} \\{\quad {d\quad L}} \\\quad\end{matrix}$

[0080] where dF is the force required to produce an increase of lengthdL in the of the structure.

[0081] By way of example of the calculations above, the amplitude of thefluctuating load torque T may be 10 Nm (zero to peak), and the radius ofthe rotor 12 may be 50 mm. This gives a maximum force F required toprovide the required fluctuating corrective torque of F=200N. In theexample discussed, the required change in span length is obtained bydividing the tension of 200 N by the stiffness coefficient k, which forexample for a typical belt may be 400 N/mm. This gives requiredamplitude of elongation and contraction of the timing belt of 0.5 mm(zero to peak).

[0082] The next step is to calculate the eccentricity required toprovide this length of elongation and contraction at a timing stage whenthe major axis 20 of the ellipse is set at θ=135° as shown in FIG. 1. Atheoretical calculation of this value is difficult to achieve, so thatthe calculation of the eccentricity is arrived at by the equivalent of a“look-up” table. This is done by producing and recording data to relateempirically a series of values of (i) the divergence from circular ofthe protruding and receding portions of the non-circular profile and(ii) the resulting amplitude of the periodic elongation and contractionof the drive span. The required eccentricity is then selected from thedata to give the required amplitude of the periodic elongation andcontraction of the drive span.

[0083] The data bank which is produced, to provide the “look-up” tableconsists of a table of values of the amplitude of elongation andcontraction of the drive span 10A and 10B, for various values of theeccentricity of the oval profile 19 along the major axis. Examples ofsuch data are given in the following table, Table 1. The referencecircle used for comparison is a circle having a diameter equal to theaverage of the major axis length 20 and the minor axis length 21. Theeccentricity of the oval profile 19 can be determined, in the exampleshown, by considering the divergence of the outline from the referencecircle at the major axis 20. Difference between selected Amplitude ofperiodic oval reference outline elongation and contraction and referencecircle of drive span 10A, 10B 0.5 mm 0.25 mm 1.0 mm 0.49 mm 1.5 mm 0.74mm

[0084] This table may be derived for example by producing a computersimulation of the oval profile 19, and stepping this through a series ofangular advancements of, say one tooth at a time, for example as shownin FIGS. 15, 16 and 17. For each of these steps, the computer simulationis arranged to provide an indication of the elongation or contraction ofthe equivalent drive span 10A, 10B, for a particular length of majoraxis giving the radius 20A. On the computer simulation, the referenceradius 20A is then varied, and a further series of data are produced forthe new radius 20A. The purpose of stepping the profile through thepositions shown at FIGS. 15, 16 and 17, is to determine empirically theposition at which the maximum extension of the corresponding drive span10A, 10B takes place. Having determined that, the appropriate data isextracted, for the maximum length of the span 10A, 10B, which is setagainst the corresponding eccentricity of the reference radius 20A.FIGS. 15, 16 and 17 show how the amplitude of elongation may bedetermined by using virtual prototyping.

[0085]FIGS. 5a to 5 d show different combinations of crankshaft andcamshaft sprockets for 4-cylinder and 3-cylinder engines. FIGS. 6a to 6d show different combinations of crankshaft and camshaft sprockets for6-cylinder, 8-cylinder and 2-cylinder engines.

[0086]FIG. 7a shows the amplitude of camshaft torsional vibrations indegrees of rotary vibration versus the engine speed in rpm for a roundcrankshaft sprocket. FIG. 7b shows the amplitude of camshaft torsionalvibrations in degrees of rotary vibration versus the engine speed in rpmfor an oval crankshaft sprocket. FIG. 7b shows that the torsionals aresignificantly reduced. Only torsionals coming from the crankshaftremain. The resonance has been cancelled.

[0087]FIG. 8a shows the tight side tension fluctuation versus the enginespeed in rpm for a round crankshaft sprocket. FIG. 8b shows the tightside tension fluctuation versus the engine speed in rpm for an ovalcrankshaft sprocket. FIG. 8b also shows that resonance has beencancelled. Tension fluctuations are still present in the whole rpmrange, but they need to be there to provide cancelling torque.

[0088]FIGS. 9a and b show the tight side and slack side tensionfluctuations over one revolution of the round crankshaft sprocket at1500 rpm. FIGS. 10a and b show the tight side and slack side tensionfluctuations over one revolution of the round crankshaft sprocket at thesystem resonance (2500 rpm). FIGS. 11a and b show the tight side andslack side tension fluctuations over one revolution of the roundcrankshaft sprocket at 3500 rpm.

[0089]FIG. 12 shows the camshaft torsional vibrations for a roundcrankshaft sprocket presented as a spectral analysis where:x-axis=harmonics orders; y-axis=engine rpm; and z-axis=amplitude of thecamshaft torsional vibrations.

[0090]FIG. 13 shows the camshaft torsional vibrations for an ovalcrankshaft sprocket presented as a spectral analysis where:x-axis=harmonics orders; y-axis=engine rpm; and z-axis=amplitude of thecamshaft torsional vibrations. Only second order torsionals areeliminated by the oval profile. Using a more complex profile, as shownin FIG. 14 will cancel simultaneously second and fourth ordertorsionals.

[0091]FIGS. 14a and 14 b show, in greatly exaggerated form, how anon-circular profile 19 of one of the rotors in a synchronous driveapparatus embodying the invention can be shaped to accommodate twodifferent orders of torsional fluctuations in the torque of a rotaryload assembly. FIG. 14 consists of two FIGS. 14a and 14 b. FIG. 14ashows in curve 110 a second order fluctuating load torque, equivalent tothe second order peak shown in FIG. 12. The curve 111 shows a fourthorder fluctuating load torque equivalent to the fourth order peak shownin FIG. 12. Curve 112 shows the combined fluctuating load torque on therotary load assembly.

[0092] In FIG. 14b there is shown at 19A in greatly exaggerated form agenerally oval profile suitable for use on a crankshaft rotor 11 in FIG.1, having protruding portions 22 and 23. These protruding portionsproduce a corrective fluctuating load torque which can be applied tocancel the second order fluctuating load torque 110 in FIG. 14a. Asecond profile indicated at 19B is shaped to have four minor protrudingportions which, if it were to be used as a profile of crankshaftsprocket 11, would produce a corrective torque equivalent to the fourthorder fluctuating load torque 111 in FIG. 14a. In FIG. 14b, anon-circular profile embodying the invention is indicated at 19C, whichis a combination of the two profiles 19A and 19B. The combined profile19C has two major protruding portions, and two minor protrudingportions. The combined profile 19C produces a fluctuating correctivetorque which can be made to cancel the combined fluctuating torque 112shown in FIG. 14a.

[0093] Thus in FIG. 14, there is shown a modification of the oval rotorin which additional minor protruding portions of the profile areprovided. The reason for this is to take account of fourth orderharmonic torsional vibrations which are illustrated in FIGS. 12 and 13.In FIG. 12, there is shown the torsional vibrations which arise from thesecond, fourth and sixth order harmonics, with a synchronous driveapparatus having a circular crankshaft sprocket. FIG. 13 shows thetorsional vibrations remaining after use of an oval crankshaft drivesprocket in accordance with the invention. It will be seen that thefourth order harmonic torsional vibrations remain. These vibrations canbe reduced or eliminated by providing on the non-circular profile of thecrankshaft sprocket additional protruding portions. The minor protrudingportions are of lesser extent than the major protruding portions, andare arranged to produce lesser fluctuating corrective torque patterns inthe torque applied to the second rotor, to reduce or substantiallycancel the fourth order fluctuating load torque presented by the rotaryload assembly.

[0094] Returning now to a general consideration of the operation ofembodiments of the invention, it is known to provide in a synchronousdrive system for an internal combustion engine a crankshaft sprocket ofoval profile. The present invention provides for the correct selectionof the eccentricity and the timing of the non-circular profile, to bethat which advantageously cancels or reduces the fluctuating load torquein the load assembly, rather than endeavouring to equalise the tensionin the drive belt, has as been done in the prior art arrangements.

[0095] The invention can be understood by considering Newton's secondlaw, that the presence of an unbalanced force will accelerate an object.For linear examples this provides:

Acceleration=Force/Mass

[0096] In rotary motion:

Acceleration=Torque/Inertia

[0097] In an ordinary internal combustion engine the torque from thevalve train or diesel fuel pump fluctuates, causing the speed tofluctuate, causing angular displacement to fluctuate (also known astorsional vibration). By using an ellipsoidal crankshaft sprocket thatis pulling the belt (at appropriate instant) additional torque can becreated that has such amplitude and phase that the combined torqueacting on the camshaft is zero. Absence of torque means absence ofacceleration by first Newton's law. Absence of acceleration meansabsence of speed fluctuations, which means that no torsionals arepresent.

[0098] The opening and closing of the intake and exhaust valves is asource of torque fluctuations. These torque fluctuations cause thecamshaft to be inflicted with speed fluctuations, which in turn, causesangular position fluctuations otherwise know as torsional vibrations.The best cure for that behaviour is to attack the cause right at thesource by introducing another torque acting on the camshaft i.e.removing torque fluctuations at the camshaft. One way of doing it is touse the oval sprocket at the crankshaft. The oval sprocket, whilerotating, will introduce fluctuations of span length i.e. will pull andrelieve two times per one crankshaft revolution. When the tight side isbeing pulled, the slack side is relieved and vice versa. Pulling andrelieving the belt means that a new, additional torque is generated atthe camshaft. If this new torque is of appropriate amplitude and phaseit can balance the first torque from the valve train. Absence of torquefluctuations means absence of speed fluctuations and therefore absenceof torsionals.

[0099] In embodiments of the invention, when the torsional vibrations inthe camshaft are eliminated the belt tension still varies. Indeed it isthe variation in tension in the belt, which causes the torsionalvibrations in the camshaft to cease. In the prior art, the objective issaid to be the removal of tension variation in the belt, which is notwhat is needed to remove torsional vibration in the camshaft. The objectis to remove the variation in speed of the driven sprocket, which iscaused by variation in torque load in the driven sprocket. This is doneby varying the tension in the belt during the cycle of the drivensprocket. At a time of increase of torque load on the driven sprocket,there must be an increase in tension in the belt. At moment whenincrease in tension is required the effective length of the span must beincreased. This is achieved by having the oval positioned so that thelong axis is moving from a position perpendicular to the hub load, toposition along the hub load direction. At the moment when decrease intension is required the effective length of the span must be decreased.This is done while the major axis moves from vertical to horizontal.

What is claimed is:
 1. A synchronous drive apparatus, comprising: a continuous-loop elongate drive structure (10) having a plurality of engaging sections (15); a plurality of rotors comprising at least a first and a second rotor (11, 12), the first rotor (11) having a plurality of teeth (16) for engaging the engaging sections (15) of the elongate drive structure (10), and the second rotor (12) having a plurality of teeth (16) for engaging the engaging section (15) of the elongate drive structure (10); a rotary load assembly (26) coupled to the second rotor (12); the elongate drive structure being engaged about the first and second rotors, the first rotor (11) being arranged to drive the elongate drive structure (10) and the second rotor (12) being arranged to be driven by the elongate drive structure (10), and one of the rotors having a non-circular profile (19) having at least two protruding portions (22, 23) alternating with receding portions (24, 25), the rotary load assembly (26) being such as to present a periodic fluctuating load torque when driven in rotation; characterised in that the angular positions of the protruding and receding portions of the non-circular profile (19) relative to the angular position of the second rotor (12), and the magnitude of the eccentricity of the non-circular profile (19), are such that the non-circular profile applies to the second rotor an opposing fluctuating corrective torque (104) which reduces or substantially cancels the fluctuating load torque (103) of the rotary load assembly (26).
 2. A synchronous drive apparatus according to claim 1, in which the non-circular profile (19) is such as to produce the said opposing fluctuating corrective torque by periodic elongation and contraction of the spans of the elongate drive structure (10) adjoining the rotor on which the non-circular profile (19) is formed, the elongate drive structure having a drive span (10A, 10B) on the tight side of the rotor on which the non-circular profile (19) is formed, the angular position of the non-circular profile (19) being within +/−15 degrees of an angular position for which a maximum elongation of the said drive span (10A, 10B) coincides with a peak value of the fluctuating load torque (103) of the rotary load assembly (26).
 3. A synchronous drive apparatus according to claim 2, in which the angular position of the non-circular profile (19) is within +/−5 degrees of the angular position for which a maximum elongation of the said drive span (10A, 10B) coincides with a peak value of the fluctuating load torque (103) of the rotary load assembly (26).
 4. A synchronous drive apparatus according to claim 2, in which the angular position of the non-circular profile (19) is that for which a maximum elongation of the said drive span (10A, 10B) substantially coincides with a peak value of the fluctuating load torque (103) of the rotary load assembly (26).
 5. A synchronous drive apparatus according to claim 1, in which the magnitude of the eccentricity of the non-circular profile is such that the fluctuating corrective torque (104) has an amplitude in the range of 70% to 110% of the amplitude of the fluctuating load torque (103) at a predetermined selected set of operating conditions of the synchronous drive apparatus.
 6. A synchronous drive apparatus according to claim 5, in which the said range consists of 90% to 100% of the amplitude of the fluctuating load torque (103).
 7. A synchronous drive apparatus according to claim 5, in which the amplitude of the fluctuating corrective torque (104) is substantially equal to the amplitude of the fluctuating load torque (103).
 8. A synchronous drive assembly according to claim 1, in which the amplitude of the fluctuating load torque (103) of the rotary load assembly (26) is substantially constant, and the magnitude of the eccentricity of the non-circular profile (19) is such that the fluctuating corrective torque (104) has an amplitude in the range of 70% to 110% of the amplitude of the fluctuating load torque (103).
 9. A synchronous drive apparatus according to claim 8, in which the said range consists of 90% to 100% of the amplitude of the fluctuating load torque (103).
 10. A synchronous drive apparatus according to claim 8, in which the amplitude of the fluctuating corrective torque (104) is substantially equal to the amplitude of the fluctuating load torque (103).
 11. A synchronous drive assembly according to claim 1, in which the amplitude of the fluctuating load torque (103) of the rotary load assembly (26) varies, and the magnitude of the eccentricity of the non-circular profile (19) is such that the fluctuating corrective torque (104) has an amplitude in the range of 70% to 110% of the amplitude of the fluctuating load torque when determined at conditions such that it is a maximum.
 12. A synchronous drive apparatus according to claim 11, in which the said range consists of 90% to 100% of the amplitude of the fluctuating load torque (103) when determined at conditions such that it is a maximum.
 13. A synchronous drive apparatus according to claim 11, in which the amplitude of the fluctuating corrective torque (104) is substantially equal to the amplitude of the fluctuating load torque (103) when determined at conditions such that it is a maximum.
 14. A synchronous drive assembly according to claim 1, in which the amplitude of the fluctuating load torque (103) of the rotary load assembly varies, and the magnitude of the eccentricity of the non-circular profile (19) is such that the fluctuating corrective torque (104) has an amplitude in the range of 70% to 110% of the amplitude of the fluctuating load torque (103) when determined at the natural resonance frequency of the apparatus.
 15. A synchronous drive apparatus according to claim 14, in which the said range consists of 90% to 100% of the amplitude of the fluctuating load torque (103) when determined at the natural frequency of the apparatus.
 16. A synchronous drive apparatus according to claim 14, in which the amplitude of the fluctuating corrective torque (104) is substantially equal to the amplitude of the fluctuating load torque (103) when determined at the natural frequency of the apparatus.
 17. A synchronous drive apparatus according to claim 1, in which the magnitude of the eccentricity of the non-circular profile (19) is such as to produce a periodic elongation and contraction of said drive span given by the formula: $L = \frac{T}{rk}$

L=the amplitude of the periodic elongation and contraction of the said drive span (10A, 10B); T=the amplitude of the fluctuating load torque (103) of the rotary load assembly at a predetermined selected set of operating conditions of the synchronous drive apparatus; r=the radius of the second rotor: k=the stiffness coefficient of the elongate drive structure (10) defined as $\begin{matrix} {k = \quad {dF}} \\ {\quad {d\quad L}} \\ \quad \end{matrix}$

 where dF is the force required to produce an increase of length dL in the length of the structure.
 18. A synchronous drive apparatus according to claim 17, in which the said operating conditions are such that the amplitude of the fluctuating load torque (103) of the rotary load assembly (26) is substantially constant.
 19. A synchronous drive apparatus according to claim 17, in which the amplitude of the fluctuating load torque (103) of the rotary load assembly (26) varies, and T=the amplitude of the fluctuating load torque of the rotary load assembly (26) determined at conditions when it is a maximum.
 20. A synchronous drive apparatus according to claim 17, in which the amplitude of the fluctuating load torque of the rotary load assembly (26) varies, and T=the amplitude of the fluctuating load torque (103) of the rotary load assembly determined at the natural resonance frequency of the synchronous drive apparatus.
 21. A synchronous drive apparatus according to claim 1 in which the said non-circular profile (19) is provided on the first rotor (11).
 22. A synchronous drive apparatus according to claim 1, in which the said non-circular profile (19) is provided on the second rotor (12).
 23. A synchronous drive apparatus according to claim 1, in which the non-circular profile (19) is provided on a third rotor (14).
 24. A synchronous drive apparatus according to claim 23, in which the third rotor (14) comprises an idler rotor urged into contact with the continuous loop elongate drive structure (10), the third rotor (10) having a plurality of teeth (16) for engaging the engaging sections (15) of the elongate drive structure.
 25. A synchronous drive apparatus according to claim 1, when installed in an internal combustion engine, the said first rotor (11) comprising a crankshaft sprocket.
 26. A synchronous drive apparatus according to claim 25, in which the internal combustion engine is a diesel engine, and the said rotary load assembly (26) comprises a rotary fuel pump.
 27. A synchronous drive apparatus according to claim 26, in which the magnitude of the eccentricity of the non-circular profile is such that the fluctuating corrective torque (104) has an amplitude substantially equal to the amplitude of the fluctuating load torque (103) when determined at conditions of maximum delivery of the fuel pump.
 28. A synchronous drive apparatus according to claim 25, in which the internal combustion engine is a petrol engine and the rotary load assembly (26) comprises a camshaft assembly.
 29. A synchronous drive apparatus according to claim 28, in which the fluctuating load torque (103) of the camshaft assembly is substantially constant throughout the rev range of the engine, and the amplitude of the fluctuating corrective torque (104) is substantially equal to the amplitude of the fluctuating load torque (103).
 30. Apparatus according to claim 1, in which the non-circular profile has at least two reference radii (20A, 20B), each reference radius passing from the centre of the rotor on which the non-circular profile (19) is formed and through the centre of a protruding portion (22, 23) of the non-circular profile (19), the angular position of the non-circular profile (19) being related to a reference direction (27) of the rotor on which the non-circular profile (19) is formed, the reference direction being the direction of a vector (27) that bisects the angle about which the elongate drive structure (10) is wrapped about the rotor having the non-circular profile (19), the angular position of the non-circular profile (19) being such that, when the fluctuating load torque of the rotary load assembly is at a maximum, the angular position of a reference radius (20A) is within a range of 90° to 180° from the reference direction (27) taken in the direction of rotation of the rotor on which the non-circular profile (19) is formed.
 31. Apparatus according to claim 30, in which the angular position of the reference radius (20A) being within a range of 130° to 140° from the reference direction (27) taken in the direction of rotation of the rotor on which the non-circular profile is formed.
 32. Apparatus according to claim 30, in which the angular position of the reference radius (20A) is substantially at 135° from the reference direction (27) taken in the direction of rotation of the rotor on which the non-circular profile is formed.
 33. Apparatus according to claim 1, in which the said non-circular profile (19) is a generally oval profile.
 34. Apparatus according to claim 1, in which the said non-circular profile (19) has three protruding portions arranged regularly around the rotor.
 35. Apparatus according to claim 1, in which the said non-circular profile (19) has four protruding portions arranged regularly around the rotor.
 36. Apparatus according to claim 1, in which the said protruding portions constitute major protruding portions (22, 23) and the said receding portions constitute major receding portions (24, 25), and the non-circular profile (19) includes additional minor protruding portions of lesser extent than the major protruding portions (22, 23), adapted to produce additional, minor, fluctuating corrective torque patterns in the torque applied to the second rotor (12), to reduce or substantially cancel subsidiary order fluctuating load torque presented by the rotary load assembly (26).
 37. Apparatus according to claim 1, in which the continuous-loop elongate structure (10) is toothed belt.
 38. Apparatus according to claim 1, in which the continuous-loop elongate structure (10) is a drive chain.
 39. A method of operating a synchronous drive apparatus which comprises a continuous-loop elongate drive structure (10) having a plurality of engaging sections (15); a plurality of rotors comprising at least a first and a second rotor (11, 12), the first rotor (11) having a plurality of teeth (16) engaging the engaging sections of the elongate drive structure, and the second rotor (12) having a plurality of teeth (16) engaging the engaging section of the elongate drive structure; and a rotary load assembly (26) coupled to the second rotor; one of the rotors having a non-circular profile (19) having at least two protruding portions (22, 23) alternating with receding portions (24, 25), and the rotary load assembly (26) presenting a periodic fluctuating load torque (103) when driven in rotation; the method comprising the steps of engaging the elongate drive structure about the first and second rotors, driving the elongate drive structure (10) by the first rotor (11), and driving the second rotor (12) by the elongate drive structure (10); characterised by applying to the second rotor (12) by means of the non-circular profile (19) an opposing fluctuating corrective torque (104) which reduces or substantially cancels the fluctuating load torque (103) of the rotary load assembly (26).
 40. A method according to claim 39, including: producing said opposing fluctuating corrective torque (104) by the non-circular profile (19) by a periodic elongation and contraction of spans of the elongate drive structure including a drive span (10A, 10B) on the tight side of the non-circular profile (19); and producing a maximum elongation of the said drive span (10A, 10B) at an angular position of the non-circular profile (19) which is within +/−15 degrees of an angular position for which a maximum elongation of the said drive span (10A, 10B) coincides with a peak value of the fluctuating load torque (103) of the rotary load assembly (26).
 41. A method according to claim 39, including producing a maximum elongation of the said drive span (10A, 10B) at a time which substantially coincides with a peak value of the fluctuating load torque (103) of the rotary load assembly (26).
 42. A method according to claim 39, including applying to the second rotor (12) a fluctuating corrective torque (104) which has an amplitude in the range of 70% to 110% of the amplitude of the fluctuating load torque (103) at selected predetermined conditions for which reduction or substantial cancellation of fluctuating load torque (103) is required.
 43. A method according to claim 42, including applying to the second rotor (12) a fluctuating corrective torque (104) which is substantially equal to the amplitude of the fluctuating load torque (103) at selected predetermined conditions for which reduction or substantial cancellation of fluctuating load torque is required.
 44. A method according to claim 39, in which the amplitude of the fluctuating load torque (103) of the rotary load assembly (26) is substantially constant, and the method includes applying to the second rotor a fluctuating corrective torque which has an amplitude in the range of 70% to 110% of the amplitude of the fluctuating load torque.
 45. A method according to claim 44, including applying to the second rotor a fluctuating corrective torque (104) which is substantially equal to the amplitude of the fluctuating load torque (103).
 46. A method according to claim 39, in which the amplitude of the fluctuating load torque (103) of the rotary load assembly (26) varies, and the method includes applying to the second rotor a fluctuating corrective torque (104) which has an amplitude in the range of 70% to 110% of the amplitude of the fluctuating load torque (103) when determined at conditions such that it is a maximum.
 47. A method according to claim 46, including applying to the second rotor a fluctuating corrective torque (104) which is substantially equal to the amplitude of the fluctuating load torque (103) when determined at conditions such that it is a maximum.
 48. A method according to claim 39, including applying to the second rotor a fluctuating corrective torque (104) which has an amplitude in the range of 70% to 110% of the amplitude of the fluctuating load torque (103) when determined at the natural resonant frequency of the apparatus.
 49. A method according to claim 46, including applying to the second rotor a fluctuating corrective torque (104) which is substantially equal to the amplitude of the fluctuating load torque (103) when determined at the natural resonance frequency of the apparatus.
 50. A method of constructing a synchronous drive apparatus, comprising: assembling components comprising a continuous-loop elongate drive structure (10) having a plurality of engaging sections (15); a plurality of rotors comprising at least a first and a second rotor (11, 12), the first rotor having a plurality of teeth (16) for engaging the engaging sections of the elongate drive structure (10), and the second rotor (12) having a plurality of teeth (16) for engaging the engaging section of the elongate drive structure (10); and a rotary load assembly coupled to the second rotor (12); and engaging the elongate drive structure about the first and second rotors, the first rotor (11) being arranged to drive the elongate drive structure (10) and the second rotor (12) being arranged to be driven by the elongate drive structure (10), and one of the rotors having a non-circular profile (19) having at least two protruding portions (22, 23) alternating with receding portions (24, 25), the rotary load assembly (26) being such as to present a periodic fluctuating load torque (103) when driven in rotation; characterised by the steps of determining the angular positions of the protruding and receding portions of the non-circular profile (19) relative to the angular position of the second rotor (12), and the magnitude of the eccentricity of the non-circular profile (19), to be such that the non-circular profile (19) applies to the second rotor (12) an opposing fluctuating corrective torque (104) which reduces or substantially cancels the fluctuating load torque (103) of the rotary load assembly (26).
 51. A method according to claim 50, including: arranging the non-circular profile (19) to produce the said opposing fluctuating corrective torque (104) by periodic elongation and contraction of the spans of the elongate drive structure (10) adjoining the rotor on which the non-circular profile (19) is formed, the elongate drive structure (10) having a drive span (10A, 10B) between the rotor on which the non-circular profile is formed and the second rotor, the drive span being positioned on the tight side of the rotor on which the non-circular profile is formed; and determining the angular positions of the protruding and receding portions of the non-circular profile (19) by arranging the angular position of the non-circular profile to be within +/−15 degrees of an angular position for which a maximum elongation of the said drive span (10A, 10B) coincides with a peak value of the fluctuating load torque (103) of the rotary load assembly (26).
 52. A method according to claim 51, including arranging the angular position of the non-circular profile (19) to be within +/−5 degrees of the angular position for which a maximum elongation of the said drive span (10A, 10B) coincides with a peak value of the fluctuating load torque (103) of the rotary load assembly (26).
 53. A method according to claim 52, including arranging the angular position of the non-circular profile (19) to be that for which a maximum elongation of the said drive span (10A, 10B) substantially coincides with a peak value of the fluctuating load torque (103) of the rotary load assembly (26).
 54. A method according to claim 50, in which the magnitude of the eccentricity of the non-circular profile (19) is determined by the following steps: (i) measuring the amplitude of the fluctuating load torque (103) of the rotary load assembly (26) at a predetermined selected set of operating conditions of the synchronous drive apparatus; (ii) calculating the required amplitude of periodic elongation and contraction of the said drive span (10A, 10B) by the following formula: $L = \frac{T}{rk}$

L=the amplitude of the periodic elongation and contraction of the said drive span (10A, 10B); T=the amplitude of the fluctuating load torque (103) of the rotary load assembly (26) at a predetermined selected set of operating conditions of the synchronous drive apparatus; r=the radius of the second rotor (12): k=the stiffness coefficient of the elongate drive structure (10) defined as $\begin{matrix} {k = \quad {dF}} \\ {\quad {d\quad L}} \\ \quad \end{matrix}$

 where dF is the force required to produce an increase of length dL in the length of the structure. (iii) producing and recording data to relate empirically a series of values of (a) the divergence from circular of the said protruding and receding portions of the non-circular profile (19) and (b) the resulting amplitude of the periodic elongation and contraction of the said drive span (10A, 10B); and (iv) selecting from the data the corresponding eccentricity to give the required amplitude of the periodic elongation and contraction of the drive span (10A, 10B).
 55. A method according to claim 54, in which the amplitude of the fluctuating load torque (103) of the rotary load assembly (26) is substantially constant.
 56. A method according to claim 54, in which the amplitude of the fluctuating load torque (103) of the rotary load assembly (26) varies, and is determined at conditions such that it is a maximum.
 57. A method according to claim 54, in which the amplitude of the fluctuating load torque (103) of the rotary load assembly varies (26), and is determined at the natural resonance frequency of the apparatus. 